Cleanup: We should remove unnecessary #defines and IDL attributes

[jevansaks]

We have a bunch of things in IDL files that was to support compiling in the Windows codebase. We should remove the things that aren't necessary and rename things to reduce confusion. We should:

• Simplify IDL attributes down to public, preview, internal.
• Remove WUXC_INTERFACE_NAME, WUXC_STATIC_NAME, WUXC_CONSTRUCTOR_NAME
• Remove macros for namespaces, just use the namespaces directly.

[marcelwgn]

@ranjeshj @jevansaks Would this be something that should be postponed until WinUI 3 given that those might also need to be reevaluated with regard to those attributes?

I'm a bit hesitant to make such sort of changes here, since we also need to merge with WinUI 3 at some point, and we don't want to make that more painful than it needs to be.

[ranjeshj]

@llongley @kmahone as FYI. I agree we should not do this now in WinUI2. I think this can be done as part of the effort to open source winui3.

[kmahone]

I think at least some of this might have already been done in the WinUI3 codebase. But there is certainly more to do. I agree that this should wait until WinUI3 goes OSS.